Fabrication of 1-Mbit DRAMs By Using X-Ray Lithography

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Abstract
Functional 1-Mbit DRAM devices were fabricated by using x-ray lithography. An x-ray stepper ( Nikon SX-5) which has a high brightness x-ray tube (10 kW) with a Pd rotating target was used for the fabrication of the devices. The alignment system is based upon the use of a linear diffraction grating as an alignment target. The mask-to-wafer alignment is performed by detecting the first-order diffraction beam of incident laser beams from the grating alignment target. The wafer diameter is 150 mm, the mask diameter is 75 mm, and the exposure field size is 25x25 mm2. A W-Ti alloy absorber was used as an x-ray mask absorber. Since W-Ti absorber films can be controlled to have an internal stress of less than 1 x 108 dyne/cm2, the mask distortion was less than 0.1 μm for the 15x15 mm2 field including two 1-Mbit DRAM chips. The x-ray lithography was applied to the contact process by using a novolak-based positive resist XPB. The XPB resist has high sensitivity, high resolution capability, and high dry etch resistance. The alignment error was less than 0.15μm in 3σ. A 1-Mbit DRAM with a bit yield of 99.5% was obtained. It was thus demonstrated that x-ray lithography technology has the potential for use in VLSI fabrication.

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